Methods of synthesizing DNA

1. A method of synthesizing a DNA molecule using topoisomerase-mediated ligation, by adding single nucleotides or oligomers to a DNA strand in the 3′ to 5′ direction, comprising (i) reacting a DNA molecule with a topoisomerase charged with the desired nucleotide or oligomer wherein the nucleotide or oligomer is blocked from further addition at the 5′ end, then (ii) deblocking the 5′ end of the DNA thus formed, and repeating steps (i) and (ii) until the desired nucleotide sequence is obtained.

2. The method of claim 1 wherein single nucleotides are added.

3. The method of claim 1 wherein oligomers are added.

4. The method of claim 1 wherein the step of deblocking the 5′ end of the DNA is carried out using a phosphatase enzyme.

5. The method of claim 1 wherein the DNA is double stranded and further comprising the step of the reserve chamber further comprises a ligase and ATP, to repair the DNA strand not joined by the topoisomerase.

6. The method of claim 1 wherein the topoisomerase-charged donor oligonucleotide comprises a 5′ overhang on the strand complementary to the strand bearing the topoisomerase, comprising a polyinosine sequence.

7. The method of claim 1 wherein the topoisomerase is selected from vaccinia topoisomerase and SVF topoisomerase I.

8. The method of claim 1 wherein a vaccinia topoisomerase which recognizes (C/T)CCTT is used to add deoxythymidine triphosphate (dTTP) nucleotides and a SVF topoisomerase I which recognizes CCCTG is used to add deoxyguanosine triphosphate (dGTP) nucleotides.

9. The method of claim 1 wherein a nanopore separates a chamber comprising the topoisomerase from a chamber comprising a phosphatase, wherein the nanopore is large enough to allow movement of the DNA through the nanopore by electrical attraction but is not large enough to permit movement of the topoisomerase and the phosphatase through the nanopore.

10. The method of claim 1 wherein the DNA is on a substrate or magnetic bead, such that it can be selectively exposed to or removed from the reagents as required to provide the desired sequence.

11. A method of synthesizing a DNA molecule using topoisomerase-mediated ligation, by adding oligomers to a DNA strand in the 3′ to 5′ direction, comprising (i) reacting a DNA molecule with a topoisomerase charged with the desired oligomer, then (ii) removing all of the oligomer thus added with the exception of a single base, using a type IIS restriction enzyme; (iii) dephosphorylating the 5′end of the DNA thus formed using a phosphatase; and (iv) repeating steps (i), (ii) and (iii) until the desired nucleotide sequence is obtained.

12. The method of claim 11 wherein the topoisomerase is selected from vaccinia topoisomerase and SVF topoisomerase I.

13. The method of claim 11 wherein a vaccinia topoisomerase which recognizes (C/T)CCTT is used to add deoxythymidine triphosphate (dTTP) nucleotides and a SVF topoisomerase I which recognizes CCCTG is used to add deoxyguanosine triphosphate (dGTP) nucleotides.

14. The method of claim 11 wherein a nanopore separates a chamber comprising the topoisomerase from a chamber comprising a phosphatase, wherein the DNA can pass through the nanopore by electrical attraction but the topoisomerase and the phosphatase through the nanopore cannot.

15. The method of claim 11 wherein the DNA is on a substrate or magnetic bead, such that it can be selectively exposed to or removed from the reagents as required to provide the desired sequence.

16. A method for synthesizing DNA in a nanochip comprising one or more addition chambers containing an oligonucleotide bound at the 3′ end to a topoisomerase, and one or more reserve chambers containing a phosphatase and optionally a restriction enzyme, said chambers also containing compatible buffer solution and being separated by a membrane comprising at least one nanopore, wherein the enzymes are prevented from passing through the nanopore, comprising (i) moving the 5′ end of a receiver DNA into a first addition chamber, by means of an electrical force, wherein the first addition chamber contains a topoisomerase-charged donor oligonucleotide; (ii) allowing sufficient time for the donor oligonucleotide to ligate to and thereby extend the receiver DNA; (iii) moving the 5′ end of the receiver DNA thus extended into the reserve chamber, by means of an electrical force, wherein the 5′ end of the receiver DNA is dephosphorylated by the phosphatase; and (iv) repeating the cycle of steps (i) (iii), adding oligonucleotides having the same or different informational sequence, until the desired DNA sequence is obtained.

17. The method of claim 16 wherein the enzymes are tethered to a substrate.

18. The method of claim 16 wherein the restriction enzyme is present in the reserve chamber and step (iii) further comprises cleaving of the oligonucleotide added to the receiver DNA by the restriction enzyme at a position which is one nucleotide in the 5′ direction from the oligonucleotide added by topoisomerase, so that each cycle of steps (i)-(iii) adds a single base to the 5′ end of the receiver DNA.